When use is made of nuclear reactors such as pressurized water nuclear reactors, measurements have to be carried out in the core consisting of the fuel assemblies while the reactor is operating. In particular, neutron flux measurements have to be carried out at various locations along the height of the core to gain information about the distribution of the neutron flux or of power along the axial direction of the core, which usually corresponds to the vertical direction.
The middle section of the fuel assemblies forming the core comprises an instrumentation tube into which it is possible to introduce, throughout the height of the core, a leakproof thimble inside which a neutron flux measurement probe is moved while the reactor is operating. The thimbles associated with each of the fuel assemblies in which flux measurements are being carried out must be capable of being withdrawn from these assemblies, for example when the core is recharged. These thimbles are therefore mounted so that they can move slidingly inside the guide tubes connecting the lower part of the vessel to a measuring room in which the ends of the thimbles which are opposed to the ends inserted into the core are accessible for moving the probes, for collecting the measuring signals and for moving the thimbles inside the guide tubes. The movement of the thimbles in the guide tubes is performed merely by pushing or pulling, sufficient play being provided to limit the forces to be applied to the thimble, despite the guide tubes being curved in shape over most of their extent.
In addition to the movable neutron flux measurement probes, use is also made of a set of flux measuring devices such as collectrons placed in a stationary position in the core and at various levels along its height.
Devices for measuring temperature, pressure or level also have to be used while the reactor is operating, and these are installed at various predetermined locations in the core.
In order to simplify the instrumentation of the core as well as the procedures for using and maintaining this instrumentation, it has been proposed to place all the measuring devices relating to a core assembly in which measurements are carried out inside a leakproof supporting and positioning pipe designed similarly to the thimbles which receive the movable probe. A pipe of this kind comprises a tubular external shroud in which there is mounted, in a central position and coaxially with the shroud, a sheath or thimble which makes it possible to guide a movable probe for measuring the neutron flux.
Measuring devices such as collectrons are placed in the pipe, around the central sheath and in fixed positions distributed along the length of the pipe. The pipe for supporting and positioning the measuring devices has a diameter which is slightly smaller than the diameter of a guide tube which permits it to move and connects the lower section of the vessel to the measuring room. The pipe may be positioned in the core or withdrawn from the core, from the measuring room, merely by pushing or pulling its end. All the measuring devices and the thimble in which the movable probe travels can be positioned or withdrawn in this manner. It is obviously necessary to use a central thimble and a neutron flux measurement probe which are miniaturized when compared with the prior art, the assembly instrumentation tubes being the same as those which were intended to receive only thimbles. Similarly, the passages set aside for the pipe for supporting and positioning measuring instruments in the internal structures of the reactor vessel, between the bottom of the vessel and the base of the core, as well as the guide tubes, are the same as those which were used to guide thimbles alone.
These measuring pipes or sticks must have sufficient flexibility to be capable of being moved without undue effort by being pushed and pulled inside the curved guide tubes connecting the measuring room to the bottom of the vessel. The outer shroud of these measuring pipes or sticks must, furthermore, withstand considerable forces in a fluid at a high temperature and high pressure and under irradiation. This leakproof outer shroud must, in particular, withstand the pressure of the primary cooling water exerted on its wall while the reactor is operating.
It is therefore very difficult to design an outer shroud which simultaneously possesses sufficient flexibility properties to be readily moved in the guide tubes and satisfactory mechanical strength properties under the conditions of use which prevail in the nuclear reactor. In particular, it is very difficult to find a material capable of having sufficient characteristics to form a thin tubular shroud which is subjected to high mechanical stresses in the reactor vessel and in the guide tubes.
The problems of construction of the measuring pipe are increased further by the fact that this pipe must permit the positioning of a sheath or thimble in its middle section and of measuring devices in a fixed position and distributed along its length around the central sheath.
Furthermore, some measurements, such as the measurements of temperature, pressure or liquid level in the reactor core are very difficult to perform properly or are even impossible so long as use is made of measuring means which are separated by a continuous wall from the primary fluid of the reactor.